NFS: Add nfs_set_page_dirty()

We will want to allow nfs_writepage() to distinguish between pages that
have been marked as dirty by the VM, and those that have been marked as
dirty by nfs_updatepage().
In the former case, the entire page will want to be written out, and so any
requests that were pending need to be flushed out first.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_page.h

@@ -30,6 +30,7 @@
 #define PG_BUSY			0
 #define PG_NEED_COMMIT		1
 #define PG_NEED_RESCHED		2
+#define PG_NEED_FLUSH		3
 
 struct nfs_inode;
 struct nfs_page {